Relativistic X-ray Sources

The research on Femtosecond x-ray radiation sources based on laser plasma accelerators has now shown considerable progresses during the past 5 years. We essentially worked on the Betatron source. Since its discovery in 2004 at LOA, the source has been widely developed and characterized. The flux and energy of the source were improved using multi 100 TW class laser. However, the efficiency of the mechanism was never improved and, even more importantly, the source remained unstable and unreliable for applications like time-resolved studies. In the past few years, we demonstrated methods that considerably improve the stability and the efficiency of a Betatron radiation source. In addition, we performed the first femtosecond x-ray absorption experiment using Betatron radiation. In this highly competitive field of research we studied the femtosecond structural dynamic of a copper sample brought in the Warm Dense Matter regime. We briefly discuss these progresses below.

– Improvement of the stability of the Betatron source: All the features of the Betatron radiation depend on the electrons orbits. Using ionization-induced injection in a gas mixture, the orbits of the relativistic electrons emitting the radiation become reproducible and controlled. As a result we observed that both the signal and beam profile fluctuations are significantly reduced and that the beam pointing varies by less than a tenth of the beam divergence. In addition, the radiation becomes polarized. The polarization ratio reaches 80%, and its axis follows the laser polarization. The Figure below represents example of consecutive x-ray beam profiles obtained with transverse (as used so far) and ionization injection.

X-ray profiles for different gas compositions. (a) Angular profile of the X-ray beam for four consecutive shots in pure helium and in the gas mixture (He+1% N2). The color scale is the same for all images.

 


– Improvement of the efficiency of the Betatron source: The key for the improvement of Betatron source efficiency relies in the control of the electron orbits. The efficiency of the Betatron source increase if the energy of the electron is increased and the oscillation period is decreased. However, in a laser plasma accelerator, these parameters are linked so that the electron energy increases together with the oscillation period.  We explored several methods to dissociate acceleration and wiggling. We demonstrated that decoupling is possible when two gas jets are used. In the first jet, the gas density is optimized to produce energetic electrons. In the second jet, the density is much higher to produce radiation. This method allows to increase the flux by a factor up to 3. In 2018, we demonstrated that an upward density ramp is much more efficient. The Betatron radiation signal above 2 keV was increased by a factor up to 20. Theses novel results are being analyzed. We anticipate a broad impact of the source, as its remarkable performance opens the way for new applications.

– Femtosecond x-ray absorption spectroscopy: In 2017 we gathered a team of scientists with complementary expertise from LOA, CEA and CELIA and we successfully performed the first femtosecond XANES (X-ray Absorption Near-Edge Spectroscopy) experiment. We measured the femtosecond dynamic of the Copper L-edge brought to Warm Dense Matter (WDM) conditions. A clear spectral feature (pre-edge) was observed as a result of the ultrafast electron temperature increase. The temporal resolution was evaluated to 75 ± 25 fs, mainly limited by the geometry of such proof-of-principle experiment. This first experiment demonstrates the great potential of the Betatron source and open unprecedented possibilities for femtosecond X-ray absorption. Our project aims at developing this novel class of experiments.

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